"To try to say who was the greatest would be an injustice to so many others," said Marv Levy, the former Buffalo Bills coach who recently retired from his position as the team's general manager.

Levy, Art Rooney Jr., Ken Meyer, Zeke Bratkowski, Dick Haley, Larry Kennan and James Harris have been watching, coaching or playing quarterback at the college and pro levels for a combined 321 years heading into 2008. (See credentials here.)

Their insights helped ESPN.com produce a top 10 list for the ages, even as panelists struggled to single out just 10 from a long list of strong candidates.

"My top 10 might be turning into a top 20," said Haley, a New York Jets personnel consultant and former NFL defensive back with more than 40 years of scouting experience, including a 20-year stretch with the Pittsburgh Steelers that began in 1971.

Troy Aikman, Sammy Baugh, Norm Van Brocklin, Bart Starr and Bobby Layne received top-five votes from individual panelists without gaining enough traction to finish among the top 10. Joe Namath, Steve Young and Sid Luckman each drew more than one top-10 vote.

Leaving off Aikman seemed particularly difficult given his overall skills and championship success. But some panelists felt Graham needed to be on the list because he was so far ahead of his time (in addition to his seven championships in 10 seasons from 1946 to 1955). Fouts also commanded considerable respect, not only for production but for the all-out manner in which he attacked the game.

"It's unfair to name the best because some other guys were just as good in their own situations," said Bratkowski, a former All-American quarterback at Georgia who played under George Halas and Vince Lombardi in the NFL.

Brady's accuracy, nearly flawless mechanics and ability to win championships without a supporting cast of offensive all-stars separated him from other current players, although not significantly from Manning.

"Brady may be the most accurate thrower I've ever seen," said Kennan, a longtime quarterbacks coach who collected a Super Bowl ring with the Los Angeles Raiders following the 1983 season.

"I'm not sure Manning isn't the re-creation of Unitas," Levy said.

Favre was the only other active player to earn a spot in the top 10. His unmatched durability and production mostly offset concerns about unfortunate decision-making. "Quarterbacks all have a mind of their own," said Kennan, who ranked Favre among the 10 best, "but he has taken it to new levels."

Most panelists ranked accuracy first on the list of traits they most value in a quarterback. Intelligence, work ethic, velocity, toughness, leadership, mechanics and a quick release also were important. Mobility was a plus, not a necessity. But playing the position well also requires a moxie not easily measured or explained.

"I've always had an expression I've carried with me a long way," said Meyer, who coached Namath and Ken Stabler at Alabama before coaching in the NFL for 22 seasons. "Talk about a good quarterback, and I say, 'Well, he can play cards.' There are some guys [who] can play cards, some guys [who] cannot play cards. All the good quarterbacks I've ever been around, they could play cards."

Harris, the Jacksonville Jaguars' vice president of player personnel and an NFL quarterback from 1969 to 1981, ranked toughness high on his list.

"Obviously, you need to have the physical tools," Harris said. "Once you identify those, toughness, making good decisions, work ethic and one's ability to make plays with the game on the line stand out right off the top."

Panelists agreed to submit rankings as long as their choices would remain private. ESPN.com adjusted for bias, taking note when a panelist assigned high rankings to former associates. Levy naturally favored Jim Kelly from their days together in Buffalo.

Meyer, who rates quarterback prospects for the Kansas City Chiefs while in semiretirement, remains partial to Namath and Stabler. Bratkowski backed up Starr with the Green Bay Packers, and he still has Starr's back.

Rooney, whose family owns the Steelers, has a strong affinity for Bradshaw.

A former longtime personnel evaluator, Rooney rated his 10 greatest quarterbacks across 14 categories, sending his hand-written breakdowns by mail. Levy, Rooney, Bratkowski and Kennan provided lists of their top 10 all-time quarterbacks, in preferred order. Haley singled out 10 without as much regard for order, but he spoke at length about all 10 and numerous others, providing insights that proved helpful. Harris, reached between Senior Bowl practices in Mobile, Ala., provided his top six. He also evaluated 50 quarterbacks across several key categories.

Meyer, Bratkowski, Kennan, Levy and Haley spoke at length about quarterbacks and what makes them great. Meyer declined to provide a top 10 list, but all panelists placed Brady among the greats, assuming he remains productive.

ESPN.com weighed their contributions, balancing rankings with anecdotal evidence, in an effort to create the best possible rankings. We sift through their contributions below.

ACCURACY

Where Brady fits:Experts ranked him among the five most accurate passers; some thought he might be the most accurate in NFL history.

Quarterbacks can be many things, but if they're not accurate, they won't last long.

"I've seen so many strong-armed guys go through our league and never make a dent because they just were not accurate enough and it took them too long to throw it," Haley said.

Accuracy mattered long before Bill Walsh's short-passing offense took hold in San Francisco nearly 30 years ago, but the proliferation of similar offenses has heightened its value. Walsh's offense relied more heavily on receivers gaining yards after the catch. That meant quarterbacks needed to hit receivers in stride.

Few passers could place a ball more perfectly than Walsh's hand-picked quarterback. In 1989, Montana completed 70.2 percent of his passes while averaging 9.1 yards per attempt, nearly a yard more per attempt than Brady has averaged this season.

"On a hook route, which the West Coast offense is noted for, there are times when you need to hit the left shoulder or the right shoulder because the defender is close on the other shoulder, so you throw it a little bit inside or a little bit outside," Kennan said. "And I thought Montana was phenomenal at that."

"Another route that is really difficult to throw is when the running back runs parallel to the line of scrimmage and you have to turn and throw it to him in the backfield and make it an accurate throw," Kennan said.

"Montana was uncannily accurate with that. He threw it about a foot in front of the receiver, who caught it on the dead run and gained 15 yards, where if he threw it a little bit behind him, it would have gained 3 yards."

VELOCITY

Where Brady fits:Experts lauded Brady for adjusting velocity as needed. They did not consider him a hard thrower in the tradition of Bradshaw or Elway.

Bradshaw and Elway might have thrown harder than any of the other great quarterbacks. Aikman could, and Favre can, fire downfield without many limitations.

"They could throw it as far as they wanted, as hard as they wanted, with very little effort," Haley said.

Receivers who caught Elway's passes against their chests instead of with their hands suffered markings left by the tip of the ball. The so-called "Elway cross" became part of the quarterback's legend.

Bradshaw once threw a pass in practice that bounced off the turf, traveled off the field and struck Rooney in the lower leg. The impact, although dampened by having struck the ground, left a nasty welt.

"I've never met a quarterback who doesn't say, 'Oh, I could have gotten that one in there,'" Meyer said. "Bradshaw was a guy who could throw it in there and get it in there. He was a powerful, powerful thrower."

Velocity means more than simply throwing the ball hard or over long distances. Haley recalled watching Marino come up surprisingly short during a televised skills competition featuring deep throws. Not that it mattered.

"Throwing from point A to point B, 20 or 25 yards, probably nobody got it there quicker because [of] the release quickness and the velocity through the short and medium areas," Haley said. "If you threw it early like Stabler or Marino, they didn't have to throw it 70 yards. They threw it when it had to be thrown, and that's what it's all about."

QUICK RELEASE

Where Brady fits:Brady gets rid of the ball quickly and his motion is efficient, experts said, but they did not consider him one of the great quick-release quarterbacks.

Marino and Namath set the standard in this category, which Kennan defined as "the time between when you make the decision to throw and the ball actually leaves your hand."

Quarterbacks with quick releases can hold the ball longer, giving their receivers more time to get open. Just as Joe Louis could knock out an opponent with a 6-inch punch, quarterbacks with quick releases can fire the ball without much of a windup.

"When Marino came out in the draft, the same year that Elway and Kelly and that group came out, a lot of people said, 'Boy, he's got a funny motion, and we're going to have to change it,'" Kennan said. "He had the ultimate quick release, and you couldn't sack him."

INTELLIGENCE

Where Brady fits: Experts gave Brady high marks in this category, but he has plenty of company.

Levy, who earned an advanced degree from Harvard, described the Columbia-educated Luckman as a brilliant man who made the T-formation what it has become.

Numerous others have played the position with intelligence, but brainpower doesn't always translate to football smarts.

"It isn't test score, it isn't book intelligence," Haley said. "Some of those guys, they've just got a feel for the whole thing, and they read [defenses] faster than anybody; they throw it quick, and it's out and it's there, and nobody has moved on the other side of that line."

MECHANICS

Where Brady fits:Experts struggled to find a quarterback with better mechanics. Brady clearly ranks among the very best.

Mechanics aren't everything. Layne, Baugh and more than a few old-timers delivered the ball with wild throwing motions. The passing game was in its infancy. Teams didn't employ quarterbacks coaches.

A low release point didn't stop Sonny Jurgensen from passing for more than 30,000 yards during a Hall of Fame career with the Philadelphia Eagles and the Washington Redskins. Favre sometimes delivers the ball with accuracy despite stepping awkwardly to the side or retreating. Marino could do that, too.

They are the exceptions.

"If you look at the great ones, that ball is in the middle of their chest [when they drop back and prepare to throw]," Bratkowski said. "The top of the ball is at the top edge of the numbers because now they can athletically move in any direction."

Brady's mechanics might be as sound as those of any quarterback of any era, experts agreed. Kennan singled out Ken Anderson and Steve DeBerg as quarterbacks known for strong fundamentals. Montana, Young and Starr also stood out as technicians, as did Kelly.

"I have recall of Joe Montana throwing that touchdown pass in that two-minute thing against the [Cincinnati Bengals] in the Super Bowl, a slant route to John Taylor, who was in the slot," Bratkowski said.

"Every time I looked at it, I looked at Joe's feet, and he came off of his foot rhythm and threw the ball perfect for a touchdown. That's what it takes, time after time after time. You deviate, and you are in trouble."

ROUTE OWNERSHIP

Where Brady fits: Brady throws every route with accuracy, but experts did not single out a specific one.

Luckman, Baugh and Bob Waterfield set the early standard for the deep ball. Van Brocklin, Bradshaw, Namath and Daryle Lamonica took over from there.

Aikman's ability to throw any route perfectly produced what scouts still consider the finest predraft workout by a quarterback in NFL history, a performance that cemented Aikman's status as the first overall choice in 1989.

Elway could scramble to one side, stop, set his feet and throw back across the field before defenders could react. Levy singled out Marino for throwing comeback routes as well as anybody, but there have been others.

"John Brodie could throw sideline passes as good as anybody I've ever been associated with," Meyer said of the 1957-73 San Francisco 49ers quarterback. "He could really throw that and throw it accurately."

Fouts, working in Don Coryell's innovative offense during a Hall of Fame career with the San Diego Chargers, owned the skinny post route. Aikman and Montana also made it work beautifully.

"And, boy, is that an important route," Levy said. "Most of those quick skinny post routes come against a blitz where you use what you call sight-adjust, where if the wide receiver and the quarterback both read it, all bets are off, he runs that quick post without any audible."

The skinny post is more complicated. The receiver takes a less pronounced angle toward the post, catching the ball between 17 and 22 yards downfield. The quarterback must throw the ball high enough to avoid the linebackers and hard enough for it to arrive before the free safety blasts the receiver. He does all this while freezing the free safety with his eyes, at the expense of tracking the cornerback in coverage.

Fouts would take a quick five-step drop and throw the ball without the benefit of a hitch step, which would have thrown off the pattern's all-important timing. His intended receiver would widen slightly as he released. On the receiver's fourth outside step, he would break inside slightly, hands at the ready.

"The ball has to be thrown on time so the safety doesn't get over in time to kill the receiver," Kennan said. "It's a timing throw and it's a rhythm throw, and big, strong guys don't do it as well as guys who just understand the timing and rhythm. Fouts, he totally understood all of that."

Bratkowski recalled Fouts throwing skinny post routes "almost like a machine" during warm-ups, firing one after another to get the timing just right.

"I don't care what coverage you were playing on defense," Meyer said, "you had better make sure that you had that quick post taken away on the single-receiver side, because Dan would find it right away, and boy, he could put that ball in there."

KILLER INSTINCT

Where Brady fits:No quarterback has shown less mercy than Brady this season, but panelists did not provide detailed rankings for this category.

The great ones know when to go for the kill.

Haley recalled the time Unitas, calling his own plays, dialed up a risky slant to Lenny Moore in a short-yardage situation when the slightest misfire would have meant losing the game. The play produced a Baltimore touchdown.

"It was fourth-and-short; you think they gotta run it," Haley said. "For somebody to have the nerve to throw the ball, because they are going to lose the game if he misses it -- I said to myself, 'No one would do that. You can't afford to make those kinds of decisions in that part of the field with the game on the line.'"

You couldn't afford to make those decisions, but Unitas could. Fouts also knew when to pounce.

"I thought Dan understood tempo better than any quarterback I've ever seen," Kennan said. "He was an absolute killer when he got you on the run.

"He got in and out of the huddle quickly. He knew he had you on the run, he snapped the ball on the first snap, he threw it and he knew when he had you and he went for the jugular and he got you."

TWO-MINUTE MASTERS

Where Brady fits:He is the only quarterback in NFL history to lead game-winning drives in the fourth quarters of three Super Bowls. That includes two winning drives in the final two minutes.

Long before every NFL playbook covered every aspect of the two-minute offense, Bobby Layne willed the Detroit Lions to victory when time wasn't on their side.

"He could make two minutes last for an hour," Bratkowski said.

Football was far less complex in those days.

"Layne and Unitas were the first great two-minute guys, before it got real well-coached," Kennan said. "They just figured it out. And they willed the team to win in that two-minute drill."

Starr's memorable winning drive through sub-freezing temperatures delivered Green Bay over the Dallas Cowboys in the Ice Bowl in the 1967 NFL Championship Game. Elway's efforts during "The Drive" came to define his career as well as the Cleveland Browns' inability to win big games during the 1980s.

Montana followed suit later in the decade with the drive that beat the Bengals in Super Bowl XXIII.

"There is a certain awareness that the good ones have, of the situation and the handling of it," Bratkowski said.

WORK ETHIC

Where Brady fits:Preparation is one of his strengths. Sixth-round draft choices generally don't become future Hall of Famers without putting in effort, experts said.

Panelists were reluctant to evaluate the work ethics of quarterbacks they didn't coach or work with directly. Layne and Stabler were notorious partiers, but they also played before defenses became so complex. Their undeniable ability to "play cards" -- on and off the field -- might have carried them through.

All the great ones worked hard in their own ways.

Layne was widely known as a world-class carouser during his days with the Lions in the 1950s, but his practice habits were apparently commendable.

"He would keep anyone that would stay on the practice field afterward to catch," said Haley, who played for Pittsburgh when Layne was the Steelers' quarterback in the early 1960s. "Here he had been out all night, but he would keep those guys after, and he loved to play football."

Bratkowski holds a special appreciation for hard work after watching Starr go from 17th-round draft choice to NFL legend.

"If you don't work at it, you are not going to succeed," he said. "The Tom Bradys of the word are the first ones there and the last to leave."

MOBILITY

Where Brady fits:Panelists did not rank Brady among the elite in this category, but neither did they consider it a weakness.

Most great quarterbacks could move well enough to avoid trouble.

Some could flat-out fly.

Among the greats, Young graded out as the best pure runner. Tarkenton, Staubach and Elway were among the best scramblers.

"When the plays did not work, Staubach made something out of it," Kennan said. "I think that is the ultimate thing for a quarterback. What does a quarterback do when the play gets screwed up? That makes guys either get run out of the league or in the Hall of Fame."

Elway stood out in his ability to scramble, then strike deep downfield with accuracy. Defenders had a hard time dragging him down when he ran.

Other quarterbacks moved effectively in the pocket despite lacking breakaway speed.

"Manning is a very good quarterback at moving in a small area and getting rid of the ball," Meyer said. "I'm talking about a 5-foot-by-5-foot area. Slide, bang, and the ball is gone."

Panelists valued outstanding mobility as beneficial, but far from crucial.

CONCLUSION

At least 15 or 20 quarterbacks reasonably could qualify for inclusion on the top 10 list. Panelists struggled to differentiate between quarterbacks of divergent eras.

"You can put up a list like that, and no one can dispute it with any authority," Haley said. "Certain guys stick out in my mind, and I'm sure certain ones stick out for Kenny Meyer or other guys who have coached or scouted or looked at guys for as long as we have."

The game continues to evolve, significantly affecting the value of specific skills. Years ago, quarterbacks worked hard to perfect the reverse pivot moves needed to execute wide tosses from split backfields. They called their own plays and threw to receivers who faced bump-and-run coverage all over the field. The money wasn't always great, forcing players to hold down jobs during the offseason, but they still got together for informal workouts.

Modern-day quarterbacks face complex defenses, putting more emphasis on study habits and quick thinking. Free agency has kept quarterbacks and receivers from staying together as long, at the expense of teamwork.

For as much as the game has changed, the essence of the position has remained the same from Y.A. Tittle and George Blanda to Len Dawson, Warren Moon and the rest.

A great quarterback must be an accurate passer. He must make the right decisions under pressure. He must work at his craft. And he needs that extra something.

"It's not the techniques you have, it's not the ability to scramble or something like that," Meyer said. "Basically, it's an ability to be a winning-type person in that position."